The present invention relates to sound absorption material. More specifically, the present invention relates to a fibrous acoustical absorber used in automotive trim panels that may be tuned during its fabrication for maximum sound absorption over a wide frequency range or in a specific frequency range.
A sound absorbing material must have the proper combination of hardness, density, and air flow to absorb and attenuate sound in a desired manner. Sound energy is comprised of high and low waves of air pressure that propagate through the air and can be absorbed and attenuated through many actions or mechanisms. Many materials absorb and attenuate sound through viscous losses (i.e., the movement or shearing of air in a material) or by sound induced mechanical/kinetic energy losses in the fibers of a material that result in sound energy being converted to heat. Generally, a foam-like material will attenuate sound though viscous losses and a fibrous material will attenuate sound through the kinetic energy dissipation created by the movement of its fibers. Other materials may utilize a combination of both viscous and mechanical losses.
The porosity and stiffness of a material will affect its air flow characteristics and thus its sound absorption and attenuation characteristics. For example, a relatively stiff and nonporous material having high acoustical resistance, such as brick, will merely reflect sound, as the sound energy will not easily propagate through the brick. Conversely, a window screen with low acoustical resistance will allow large amounts of air and thus sound to quickly propagate through it, attenuating only a small amount of the sound energy.
There are many applications, including the automotive field, where sound absorption and attenuation is important to the consumer. The interior surface of a vehicle is commonly covered or lined with panels of sound absorbent material that present an aesthetically appealing appearance and also absorb or attenuate exterior sound. Molded fiber glass panels and foam liners are examples of such materials used in vehicles that are able to attenuate sound over a wide range of frequencies. Conventionally, such panels are either single or multi-layered structures of fiberglass or foam material having an outside covering layer visible in the cabin, typically of a cloth or soft material, and a backing layer of a relatively stiff material that is adhered to the interior of the vehicle cabin.
Automotive headliners have evolved from simple fabric or fiberglass covered foam to improved designs specifically adapted to provide sound absorption and attenuation in a vehicle. Yet, there remains a need for improved sound barriers that attenuate sound over a broad range of frequencies or, alternatively, may be tuned for maximum sound attenuation in a specific frequency. The present invention utilizes a fibrous pad as the sound attenuation component of a trim panel that may be tuned to attenuate sound in a specific frequency range.
In accordance with the present invention, a tunable sound absorbing fibrous pad is formed by applying a film to a fibrous sheet or batt of material using heat and pressure. The sound absorbing fibrous pad is preferably formned in a continuous process such as a webline, where linespeed, heat, and pressure may be varied. In operation, the heat and pressure applied to the fibers cause the fibers of the fibrous batt to penetrate the film and create perforations within the film and the linespeed of the process will determine the exposure time of the film to the heat and pressure. Heat from the process can xe2x80x9cblow openxe2x80x9d the perforations caused by the geometric surface of the fibrous pad and create larger holes corresponding to higher process temperatures. These variables in combination will determine the size and number of perforations in the film. The size and number of perforations control the air flow resistance of the film and thus, the sound absorbing and attenuation properties of the sound absorbing fibrous pad. Accordingly, the sound absorbing and attenuation characteristics of the sound absorbing fibrous pad can be modified during the fabrication of the sound absorbing fibrous pad by varying the process variables.
An object of the present invention is to create a sound absorbing pad that may be tuned to absorb sound in a desired frequency range. As discussed previously, the processing of the pad may be manipulated such that the pad may be tuned to absorb sound at predetermined frequencies. Specifically, the material of the present invention may be tailored to attenuate problematic sound frequencies produced in a vehicle cabin.
Another object of the present invention is to create a sound absorbing pad that has been optimized to absorb and attenuate as much sound over as wide a range of frequencies as possible for the type of material being used.
A further object of the present invention is to provide an interior trim pad that is made predominantly of synthetic fibers so that the fibrous pad can be recycled or made recyclable.